The Chilean Andes Mountains. Researchers have found uplift in part of the Laguna del Maule volcanic field (credit: Sarah Strierch / Wikimedia Commons)

The Laguna del Maule volcanic field in the Chilean Andes Mountains lies in the heart of volcano country. The region is a well-known subduction zone, where the friction of one crustal plate sliding under another heats rock to form magma. But for the last 2,000 years, Laguna del Maule has been a quiet water-filled caldera.

Now, scientists are recording rapid deformation of the land around the caldera, suggesting that a magma reservoir is inflating below the surface. Helene Le Mével of the University of Wisconsin – Madison presented a poster at the American Geophysical Union’s Fall Meeting in San Francisco Wednesday to share the findings.

Researchers at the Southern Andean Volcano Observatory monitor and archive volcanic activity throughout the region, but Le Mével said that until now, the observatory had no reason to focus on the Laguna del Maule field. “It’s such an old caldera that there’s nobody there,” she said.

Using a radar technique to measure changes in the level of the ground, Le Mével and her colleagues observed that several areas around and under the lake were rising. When they set up three GPS stations to confirm this data, the results were startling: The ground is uplifting at a rate of almost 30 centimeters per year, and is now 1.4 meters higher than it was in 2007.

Using this information, the team found that the area is organized as a ring fault – a circular pattern of vents formed as land collapsed inward during the eruptions that created the volcano’s cauldron-like shape.

The area is of particular concern, Le Mével said, because the rock in this region contains a high concentration of silicon dioxide, the tough crystalline material used to make glass. Unlike magma consisting mostly of basalt, found in the more “flowy” volcanoes of Hawaii, silicic magma very thick. It tends to build up in gluey domes under the surface, and then push upward in a violent explosion.

The scientists estimate that the horizontal reservoir of magma, called a sill, measures 7.5 by 5.5 km, and sits about 5 km underground. Not only is the sill growing, pushing up the earth above it, but its growth is also accelerating.

“At some point, a magma reservoir is always going to erupt when it’s ready, we just don’t know when,” said Le Mével. She expects to see increasing seismicity right before the eruption, which she said could still be 1,000 years away. But she hopes this new evidence will be enough to fuel further research on the processes driving the deformation. “We rarely observe large silicic systems that are likely to erupt, even if it’s not next week or next year. And so it’s going to be exciting to follow it. To see how it continues.”

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